Doors on reshus harabim (Eruvin 6b)

Someone introduced me to Rabbi Yosef Gavriel Bechhofer's blog on halacha of the daf. I found today's post interesting because I was a little confused myself on the concept of doors after learning today's daf. I'll just quote some of it here:

Halacha distinguishes between the enclosure necessary to convert a carmelis into a reshus ha'yachidaand the enclosure necessary to convert a reshus ha'rabbim into a reshus ha'yachid. The Gemara relates that Yerushalayim would have been considered a reshus ha'rabbim had its doors not been closed at night.The Poskim disagree over whether these doors had to have been closed me'd'oraysa or me'd'rabbanan. Some sources maintain that me'd'oraysa a tzuras ha'pesach would have sufficed.Others maintain that in reshus ha'rabbim delasos are required me'd'oraysa. This requirement is limited, in any event, to reshus ha'rabbim. An enclosure intended to surround an area that fall into the category of carmelis does not require doors to allow carrying therein. The rectification of tzuras ha'pesach is sufficient in such cases.

He has some footnotes and more there.